Fuel pumps that include a motor section and a pump section having an impeller that is rotated by the motor section so as to pump up and pressurize fuel from a fuel tank are well known, as disclosed in JP-A-5-187382, JP-A-5-508460, JP-A-7-167081, JP-A-2003-336558, JP-A-2005-120834 and JP-A-2004-11556.
As shown in FIG. 9, a pump section 400 includes an impeller 402, a casing cover 404, and a pump casing 406. The casing cover 404 and the pump casing 406 form a casing member, which accommodates and rotatably supports the impeller 402. The casing cover 404 has a fuel suction port (not shown), through which fuel is pumped up from the fuel tank (not shown) into fuel passages 410,411. The fuel passages 410,411 are formed as C-shaped grooves along an outer periphery of the impeller 402 in the casing cover 404 and the pump casing 406, respectively The impeller 402 is disc-shaped, and a plurality of blades and blade ditches 408,409 are alternately formed at the outer periphery of the impeller 402. When the impeller 402 rotates, fuel flows out of the blade ditches 408,409 along outside walls thereof, and flows into the fuel passages 410,411. The fuel returns to the blade ditches 408,409 from the fuel passages 410,411 along radially inside walls of the blade ditches 408,409 and flows out of the blade ditches 408,409 along the radially outside walls thereof again. After the fuel repeats the above flowing out and returning, the fuel is pressurized and forms a circulating flow 412,413, as shown in FIG. 9.
Fuel is provided considerable kinetic energy from the rotating impeller 402 in a rotation direction thereof immediately after flowing out of the blade ditches 408,409 of the impeller 402. Therefore, the component of velocity in the rotation direction of the fuel flows 412,413 is bigger. However, before the fuel in the fuel passages 410,411 returns into the blade ditches 408,409, the kinetic energy of the fuel flows 412,413 decreases because of the friction with the inner walls of the fuel passages 410,411. In other words, the component of velocity in the rotation direction of the fuel flows 412,413 is a main component of velocity in the first stage that fuel flows 412,413 in the fuel passages 410,411. On the other hand, the component of velocity in the radial direction of the fuel flows 412,413 is a main component of velocity in the later stage that fuel flows in the fuel passages 410,411. Accordingly, as fuel flows closer to the inside walls of the fuel passages 410,411 in the later stage, the flow direction of the fuel gets closer to the radial direction of the impeller 402.
As described above, when the kinetic energy of the fuel flow 412,413 decreases in the later stage, the flow direction of the fuel is forced to change largely by the radially inside walls of the fuel passages 410,411, with respect to the axis of rotation of the impeller 402, and the fuel flows into the blade ditches 408,409. As a result, the kinetic energy of the fuel flow 412,413 further decreases, that is, the pump efficiency decreases.
The efficiency of the fuel pump is expressed as the product of the motor efficiency and the pump efficiency Accordingly, when the pump efficiency improves, the efficiency of the fuel pump also improves.